Sr isotope variations in vent fluids from 9 degrees 46 '-9 degrees 54 ' N East Pacific Rise: Evidence of a non-zero-Mg fluid component

Sr isotopic variations in vent fluids from six high-temperature hydrotherma l vents from between 9 degrees 46'N and 9 degrees 54'N on the East Pacific Rise are reported. The Mg/Sr-Sr-87/Sr-86 systematics of one vent, Biovent, require the presence of a fluid component that has experienced significant Mg depletion (approximate to 50%), but has undergone very little Sr exchang e with local basalts. This fluid component accounts for 3% to 10% of the Bi ovent samples by volume. Similarities between the composition of the inferr ed fluid component and that of bore-hale fluids sampled from Hole 504B sugg est that this fluid component is modified seawater, partially reacted with basalt at temperatures less than 150 degreesC, and subsequently entrained b y high-temperature vent fluids. Previously reported vent fluid data from 12 degrees 50'N East Pacific Rise suggest that the presence of the additional lower temperature fluid component admired in Biovent samples may not be un ique among seafloor hydrothermal systems. If such a fluid component is comm on in seafloor hydrothermal systems it will need to be considered in estima tes of axial hydrothermal fluxes. If not accounted for properly, the presence of an additional fluid componen t can bias calculated end-member Sr-87/Sr-86 ratios. After correction for t he influence of the additional fluid component on the Biovent samples, the calculated end-member Sr-87/Sr-86 ratio of this vent is 0.7041. Other nearb y vents, P vent and Bio9', are slightly more radiogenic, 0.7042. A and L ve nts from the southern portion of the study area have end-member Sr-87/Sr-86 ratios that are less radiogenic, 0.7039 and 0.7037, respectively. The reas on for the greater influence of seawater-derived Sr in the more northern ve nts is not known, but it may be related to entrainment of seawater in the u pflow zone. Copyright (C) 2001 Elsevier Science Ltd.